Osmosis, Diffusion & Active Transport
Cells constantly exchange materials such as oxygen, glucose, water, and mineral ions across their partially permeable cell membranes to survive and respire. Substances move passively down a concentration gradient by diffusion or osmosis without requiring metabolic energy, or actively against a concentration gradient via carrier proteins using ATP from cellular respiration. Examiners frequently test these three transport mechanisms alongside surface area to volume ratio (SA:V) calculations and the potato cylinder osmosis required practical.
- Diffusion
- The net passive movement of particles of a gas or any solute in solution from an area of higher concentration to an area of lower concentration (down a concentration gradient) due to random kinetic motion.
- Osmosis
- The net passive movement of water molecules from a dilute solution (high water concentration / high water potential) to a more concentrated solution (low water concentration / low water potential) across a partially permeable membrane.
- Active Transport
- The movement of dissolved molecules or ions across a cell membrane from a lower concentration to a higher concentration (against a concentration gradient), using carrier proteins and energy released by aerobic respiration in mitochondria.
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Mitochondria
Function in the Cell:
Site of aerobic respiration, releasing energy (ATP) for the cell. Folded inner membrane increases surface area for enzymes.
Exact GCSE / A-Level Exam Wording:
Mark scheme trap: Never say 'produces energy'! Always write 'site of aerobic respiration, which releases energy'.
Magnification = Image Size ÷ Actual Size (I = A × M). Convert cm → mm (×10) → µm (×1000) before dividing!Key Concepts & Exam Definitions (6)
- Diffusion
- The net passive movement of particles of a gas or any solute in solution from an area of higher concentration to an area of lower concentration (down a concentration gradient) due to random kinetic motion.
- Osmosis
- The net passive movement of water molecules from a dilute solution (high water concentration / high water potential) to a more concentrated solution (low water concentration / low water potential) across a partially permeable membrane.
- Active Transport
- The movement of dissolved molecules or ions across a cell membrane from a lower concentration to a higher concentration (against a concentration gradient), using carrier proteins and energy released by aerobic respiration in mitochondria.
- Surface Area to Volume Ratio (SA:V)
- Calculated by dividing total surface area (cm²) by total volume (cm³). Single-celled organisms have a large SA:V ratio allowing rapid diffusion, whereas large multicellular organisms have a small SA:V ratio and require specialised exchange systems.
- Isotonic, Hypotonic & Hypertonic Solutions
- An isotonic solution has the same solute concentration as the cell cytoplasm (no net water movement). A hypotonic solution is more dilute (water enters, making plant cells turgid or bursting animal cells by lysis). A hypertonic solution is more concentrated (water leaves, causing plasmolysis in plant cells or crenation in animal cells).
- Adaptations of Exchange Surfaces
- Effective exchange surfaces maximise diffusion rate via four key features: a large surface area (e.g., folded villi or alveoli), a thin membrane (one cell thick for a short diffusion pathway), an efficient blood supply (in animals), and ventilation for gases to maintain a steep concentration gradient.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Define osmosis for a 3-mark GCSE Biology question.
Give one example of active transport in plants and one in humans.
List three factors that increase the rate of diffusion across a membrane.
What happens to a plant cell placed in a concentrated sugar solution?
On an osmosis graph of percentage mass change vs sucrose concentration, what does the x-axis intercept represent?
Why do root hair cells contain large numbers of mitochondria?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain why root hair cells cannot absorb mineral ions from the soil by diffusion and describe how they absorb them instead. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Mineral ions are in a lower concentration in the soil water than inside the root hair cell cytoplasm [1 mark]. 2. Diffusion only moves substances down a concentration gradient (high to low), so ions would diffuse out rather than in [1 mark]. 3. Instead, root hair cells use active transport to move mineral ions against the concentration gradient [1 mark]. 4. This process requires energy from aerobic respiration taking place in the cell's mitochondria [1 mark].
- Q2
A student places a potato cylinder of initial mass 2.40 g into a 0.6 mol/dm³ sucrose solution for 24 hours. Its final mass is 2.04 g. Calculate the percentage change in mass and explain why the mass changed. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Change in mass = 2.04 g - 2.40 g = -0.36 g [1 mark]. 2. Percentage change = (-0.36 / 2.40) x 100 = -15% [1 mark]. 3. The 0.6 mol/dm³ sucrose solution had a lower water concentration (was more concentrated) than the potato cell cytoplasm [1 mark]. 4. Therefore, water moved out of the potato cells by osmosis across their partially permeable membranes [1 mark].
- Q3
Cube A has sides of length 1 cm and Cube B has sides of length 4 cm. Calculate the surface area to volume ratio (SA:V) for both cubes and explain the biological significance of the difference. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Cube A: Surface area = 6 x (1 x 1) = 6 cm²; Volume = 1 x 1 x 1 = 1 cm³; SA:V = 6:1 [1 mark]. 2. Cube B: Surface area = 6 x (4 x 4) = 96 cm²; Volume = 4 x 4 x 4 = 64 cm³; SA:V = 96:64 = 1.5:1 [1 mark]. 3. As size increases, the surface area to volume ratio decreases [1 mark]. 4. Larger organisms (like Cube B) have a longer diffusion distance to the centre and insufficient surface area to rely on simple diffusion alone, so they need specialised exchange surfaces and transport systems [1 mark].
- Q4
Explain how the villi in the small intestine are adapted to maximise the absorption of digested food molecules. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Finger-like projections and microvilli provide a very large surface area for rapid diffusion and active transport [1 mark]. 2. The wall of the villus is only one cell thick, providing a very short diffusion pathway to the blood [1 mark]. 3. A dense network of blood capillaries maintains a steep concentration gradient by continuously carrying absorbed glucose and amino acids away [1 mark]. 4. Epithelial cells contain many mitochondria to supply energy for active transport of glucose against a concentration gradient [1 mark].
Common Exam Questions (4)
What is the main difference between diffusion, osmosis, and active transport?
Diffusion and osmosis are both passive processes that move particles down a concentration gradient without using cellular energy, whereas active transport uses energy from respiration (ATP) to pump particles against a concentration gradient (from low to high concentration). Osmosis specifically refers only to the movement of water across a partially permeable membrane.
How do you calculate percentage change in mass in the osmosis required practical?
Use the formula: Percentage change in mass = ((Final mass: Initial mass) / Initial mass) x 100. Scientists use percentage change rather than raw mass change to control for slight variations in the starting masses of different potato cylinders.
Why do multicellular organisms need specialised exchange surfaces?
As an organism increases in size, its surface area to volume ratio (SA:V) decreases dramatically. This means simple diffusion across the outer body surface is too slow to supply interior cells with sufficient oxygen and nutrients, requiring specialised exchange organs like lungs, gills, or villi.
Why must potato cylinders be blotted dry before weighing in the osmosis experiment?
Blotting the potato cylinders gently with a paper towel removes excess surface solution that did not enter the cells by osmosis. Leaving surface liquid on the cylinder would artificially inflate the mass reading and reduce accuracy.
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Related study guides
Animal & Plant Cell Structure
All living organisms are composed of cells. Plant and animal cells are eukaryotic (containing a membrane-bound nucleus and organelles), whereas bacterial cells are prokaryotic and carry their DNA free in the cytoplasm as a single circular chromosomal loop and small rings called plasmids.
Xylem, Phloem, Transpiration & Translocation
Plants use two specialised vascular tissues to transport substances between roots, stems, and leaves. Xylem vessels carry water and dissolved mineral ions upwards from the roots to the leaves in the transpiration stream, while phloem tissue transports dissolved sugars both upwards and downwards by translocation.
Alveoli, Ventilation & Gas Exchange in the Lungs
The human gas exchange system is located in the thorax, protected by the ribcage, where ventilation continuously brings fresh oxygen into millions of microscopic air sacs called alveoli and removes waste carbon dioxide. Inhalation and exhalation rely on pressure changes created by the diaphragm and intercostal muscles altering the volume of the thoracic cavity. Examiners frequently test the four structural adaptations of alveoli for rapid diffusion and the step-by-step mechanics of ventilation.